An aerofoil is a streamlined body that generates lift when moved relative to the surrounding air. Aerofoils, such as aircraft wings and horizontal tailplanes, have upper and lower geometric surfaces which together form a leading edge region and a trailing edge region.
To improve the lift performance of the aerofoil at high angles of attack, various leading edge high lift devices, such as slats and flaps, have been developed. Many aircraft have slats mounted to the wing fixed leading edge, which are deployed forwardly during take-off and landing to increase the lift coefficient of the wing. They are typically retracted during cruise to reduce drag. High lift devices increase payload and performance during take-off and landing, which gives rise to significant efficiencies for the aircraft flight as a whole. However, leading edge slats are becoming one of the greatest sources of aircraft noise during the landing phase.
A conventional aerofoil has an interrupted boundary layer at high speeds, leading to turbulent flow over the remainder of the aerofoil. This turbulent flow increases drag. A laminar aerofoil has an uninterrupted boundary layer over a much greater proportion of the aerofoil, leading to laminar flow over most of the aerofoil (typically up to around 75% of chord). This laminar flow reduces drag, and hence fuel consumption and CO2 emissions during cruise.
Laminar aerofoils require “clean” (smooth) leading edge profiles, which conflict with traditional high lift devices, such as slats. Improvements in emissions and noise reduction are two of the most important factors influencing future aircraft design, and the interrelation between high lift devices and laminar aerofoils is of particular interest.
Recent tests have shown [“The European High Lift Project EUROLIFT II-Objectives, Approach, and Structure”, R. Rudnik et al., 25th AIAA 2007-4296] that active slot blowing at the leading edge of an aircraft wing can recover a significant proportion of the performance of a slat. It is seen that up to around 70% of the high lift performance of a deployed slat can be recovered using an active continuous blowing slot.
Leading edge slot blowing has been considered in the aviation industry for many years. Slot blowing involves injecting a pressurized fluid (air) into the airflow over the aerofoil when at high angles of incidence. The slot is adapted to direct the injected fluid substantially parallel to the local surface of the aerofoil. Slot blowing energizes the boundary layer, which promotes boundary layer attachment and delays or prevents the onset of free stream flow separation. The energized boundary layer takes a fuller profile and becomes turbulent, if not already.
Prior art slot blowing investigated in the above reference to Rudnik et al. uses a spanwise slot in the upper geometric surface of the aerofoil forward (upstream) of the expected high incidence separation point. Fluid is injected into the airflow over the aerofoil in a chordwise direction from the slot towards the trailing edge. FIG. 1 shows a schematic cross section view of the aerofoil including a leading edge slot blowing device.
As can be seen from FIG. 1, the aerofoil 1 has upper and lower geometric surfaces 2, 3 on either side of the chord line 4 and which together form a leading edge region 5. Behind the leading edge region 5 is a blowing device 6. The blowing device 6 includes a piccolo tube 7 which carries a supply of high pressure air in the spanwise direction behind the leading edge. The piccolo tube 7 expels the high pressure air radially into a cavity 8, which is shaped to accelerate the air through a slot 9 extending spanwise across the upper surface 2. The slot 9 injects the air substantially parallel to the upper surface 2 in a chordwise direction towards the trailing edge, entraining the boundary layer.
Although the arrangement shown in FIG. 1 works adequately to improve the performance of the aerofoil (e.g., greater lift, later stall, and lower drag) to provide up to around 70% of the performance of a leading edge slat, the presence of the slot 9 in the upper surface 2 causes an aerodynamic step. This is undesirable for a laminar aerofoil, as the step in the leading edge region 5 will trip the laminar boundary layer when the aerofoil is at low angles of incidence, such as during cruise.